1. Field of the Invention
The present invention relates to a method and apparatus for detecting a misfired cylinder of an internal combustion engine. More particularly, this invention pertains to a method and apparatus for detecting a misfired cylinder based on a change in revolutions of an internal combustion engine.
2. Description of the Related Art
There is an apparatus for detecting a misfired cylinder of an internal combustion engine, which computes a misfire discriminating value approximately corresponding to the amount of a change in mean effective pressure based on the rotational period of the engine and discriminates a misfired cylinder on the basis of this discriminating value (refer to ISATA-Paper "Experiences with a new method for measuring the engine roughness" 1979, by R. Latsch, E. Mausner, V. Bianchi, and Japanese Patent Application No. 1-275046).
More specifically, upper top dead center (TDC) periods (180 degrees for a four-cylinder internal combustion engine), for example, are sequentially measured, and with "half" representing the TDC period of 1/2 cycle before, "old" representing the TDC period of one cycle before and "new" representing the latest TDC period, a value LU approximately corresponding to the amount of a change in mean effective pressure (amount of a change in mean effective pressure during the combustion stroke process) is calculated by substituting these TDC periods into the following equation. ##EQU1##
The value LU acquired from the above equation is compared with a slice level SL (minus value), which has been preset in accordance with operational conditions of the engine, such as an engine load and engine revolution speed. When LU falls below or equal to the slice level SL, it is assumed that the mean effective pressure has decreased due to a misfire, and that cylinder which was undergoing the combustion stroke process during the latest TDC period, "new," used in the computation of LU, is detected as a misfired cylinder. In other words, the cylinder in which the misfire has occurred is detected on the basis of a negative change in mean effective pressure, and the value LU is used for misfire discrimination, so that this LU will be considered hereunder as "a misfire discriminating value".
When consecutive misfire discriminating values LU are negative and when at least one of the values is equal to or lower than the slice level SL, the previous misfire discriminating value LU is considered to indicate the occurrence of a misfire.
In a coasting state of a vehicle, the level of the mean effective pressure significantly decreases on the whole, so that even the presence of a misfired cylinder may not clearly appear as a variation in engine revolution speed. In the coasting state, therefore, there is no difference between the discriminating values LU originating from the presence and absence of a misfired cylinder, therefore the detection of a misfired cylinder based on the misfire discriminating value LU may be impossible or very difficult.
Consequently, the previous warning of the presence of a misfired cylinder would be canceled, resulting in erroneous detection of a misfired cylinder as having the proper combustion. This deteriorates the reliability of detection of misfired cylinders.
According to the above detection of a misfired cylinder based on the misfire discriminating value LU, it is possible to detect a misfired cylinder with a high degree of precision when, for example, a single specific cylinder continuously misfires; however, there may occur an erroneous detection of a misfired cylinder when a misfire sometimes occurs in one cylinder.
When a single, noncontinuous misfire occurs as mentioned above, the rotation of the engine fluctuates after the occurrence of the misfire due to a misfire-originated reduction in mean effective pressure so that the misfire discriminating value LU computed with the rotational fluctuation reflecting thereon may greatly change to the negative side as in the case of a misfire occurring in a cylinder. Therefore, there is a possibility of detecting a unmisfired cylinder as a misfired cylinder after the occurrence of a single misfire.
Further, in a case where a misfire occurs in an unstable rotation state or a low-speed revolution state of the engine, such as during idling operation, a misfire discriminating value LU corresponding to a cylinder having normal combustion may greatly change to the negative side due to an irregular rotational change of the engine originating from the occurrence of a misfire even when a single specific cylinder is continuously misfiring. This will cause erroneous detection of the misfired cylinder.
With regard to the slice level SL which is to be compared with the misfire discriminating value LU as mentioned earlier, a slice level map having the proper values for individual operational conditions of the engine such as engine load and engine revolution speed is prepared in advance in order to cope with a change in the misfire discriminating value LU due to the conditions. Accordingly, the number for steps of finding the slice level SL matching the discriminating value increases, and a larger memory capacity is required for the slice levels SL.